Biomedical Engineering Reference
In-Depth Information
IC40B scales the summed signal and adds an of
ff
set of
2.5 V dc. IC40B's output is thus
V
2
Z
V
out
2.5
The change in impedance, which is seen between IC40D, pin 12, and ground, is given by
1
X
0
Y
0
Z
out
1
20 (5
V
Z
)
/10
The function of a pacemaker is to stimulate the heart when the heart's intrinsic pace-
maker or conduction mechanisms fail to generate an action potential. This is where the
responsive behavior of the simulator comes into play. Pacing pulse detectors are used to
sense pacing pulses generated by a pacemaker connected to the simulator. This cardiac
simulator responds to these stimuli by simulating the excitation of the heart chamber
through which the pacing pulse was received. The atrial pacing detector circuit shown in
Figure 6.25 is made up of IC7A, an inverting ampli
er with a gain of 2, a comparator,
IC8, and an inverter, IC9A. An external atrial pacing signal delivered by a pacemaker
across R73 and R74 is input to IC7A, which inverts and ampli
fi
es the pacing signal by a
factor of 2. IC8 compares the voltage on its noninverting terminal, which is set to approx-
imately 1.0 V by trimmer R43. If the inverted pacing signal detected on pin 3 of IC8 is
less than the voltage on pin 2, the output of the comparator is set high (
fi
5 V). This dig-
ital high is inverted by IC9A to a low and input to the microcontroller's RB6 input. When
the inverted pacing signal is greater than 1.0 V, the output of IC36 is low, which is
inverted by IC35E to a high. This low-to-high transition generates a “change on port B”
interrupt, which causes the microcontroller to execute the interrupt service routine code.
R40 and C25 ensure that the pacing signal has su
cient amplitude and duration to trip
the comparator. This guarantees that narrow transients do not pass through and trigger the
microcontroller. The ventricular pacing detector circuit works in the same way as the
atrial pacing detector described above, with the exception that the input pacing signal is
input across R75 and R76.
The responsive cardiac simulator was designed to exercise three-chamber pacemakers
and other devices intended to treat congestive heart failure. These sometimes deliver
bipolar current pulses to the left ventricle, and it is important to monitor the approximate
value of the positive and negative current pulses. The bar graph LED display of Figure
6.26 shows the amplitude of the left-ventricle pacing pulse in 2-mA steps. D1 and D2
steer each phase of the bipolar current pulse to the correct display circuitry. The current
for the positive phase of the left-ventricular pacing pulse
flows into the junction of
R52-R60. A minimum of 2 mA is required to turn on optocoupler IC19 and the
fl
fi
rst seg-
ment of the bar graph display.
For discussion purposes let's suppose that the pacing pulse delivered to the left ven-
tricle consists of a
4-mA current pulse followed by a
4-mA pulse. Then 2 mA would
fl
first section of the bar graph dis-
play. The voltage across R61 will forward bias the base-emitter junction of the PNP tran-
sistor in IC19 (across pins 5 and 6). Once the PNP transistor is forward biased, the input
current can also
flow through R61 and turn on the optocoupler and the
fi
fl
flow through the base-emitter junction of the NPN transistor. This cur-
rent
flow switches on the NPN transistor in the IC and allows the additional 2 mA (what's
left of our 4 mA) to
fl
flow through R52, the collector-emitter junction of the NPN transis-
tor and the second LED of the bar graph display. Current will
fl
ow through the second
LED of the bar graph display and through R53 and should be enough to forward bias the
next PNP transistor (IC12). But there would not be enough remaining current
fl
ow to turn
on the LED in the third section of the bar graph display. Each successive LED of the bar
fl
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